Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects. Examples of such devices are cells based on dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, and devices having a twisted nematic structure, such as TN cells (twisted nematic), STN cells (super-twisted nematic), SBE cells (super-birefringence effect) and OMI cells (optical mode interference). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical and thermal stability and a good stability towards electric fields and electromagnetic radiation. Further, the liquid crystal materials should have a low viscosity and should give short response times, low threshold potentials and a high contrast in the cells. Furthermore, at usual operating temperatures from about -30.degree. C. to about +80.degree. C., especially from about -20.degree. C. to about +60.degree. C., they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the above-mentioned cells. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfill different requirements depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible. For some years there has been an increased interest in actively addressed liquid crystal devices, for example TFT applications (thin film transistor) in television sets. The use of cyano compounds with positive dielectric anisotropy in such devices, however, leads in most cases to an undesired increase in the electrical conductivity.
Since liquid crystals are generally used as mixtures of several components, it is important that the components have a good miscibility with one another.